1. Field of the Invention
The present invention concerns a method and device for the thermal cleaning of objects, in particular for the removal of materials such as plastics from metal objects.
2. Discussion of the Prior Art
It is known that during the removal of plastics from metal objects by means of a thermal process, it is a problem to keep the temperature of the metal objects under strict control.
As the objects are often expensive, such as tools or machine parts, for example from the fibres industry, such as moulds in which plastics have congealed, a strict temperature control is very important, however, during a thermal cleaning, since the metal can lose its good mechanical properties when the temperature is too high, as a result of which the above-mentioned objects are damaged or lose much of their quality.
Moreover, in case of a non-homogenous temperature distribution over the metal object, damages may occur due to the ensuing tensions arising in the metal.
Cleaning furnaces are already known in which the temperature is controlled. However, the known embodiments show several disadvantages and defects.
In certain pyrolysis furnaces, the pyrolysis process is kept under control by injecting water in the furnace room when the temperature in the furnace gets too high.
A major disadvantage hereby is that the objects therein are subject to strongly varying temperatures and often start to corrode. With some objects there is also the disadvantage that, due to the periodical injection of water in certain places, the temperature in the objects to be treated is not homogenous, as a result of which damages may occur.
In other furnaces, called vacuum furnaces, a vacuum is created in the furnace during the first part of the cleaning cycle. During said first part of the cleaning cycle, the furnace room is slowly heated, such that at first, the main part of the plastics which are present on the object to be cleaned is melted off. This melted-off plastics are carried off to a receiver outside the heated furnace room, where the plastics congeal again.
Afterwards, the furnace temperature is further increased up to a value at which the plastic residues on the object to be cleaned can pyrolyse, for example at 400.degree. C. to 450.degree. C. The vacuum in the furnace is then maintained for a certain time, to make sure that the plastic residues which still remain on the object to be cleaned are pyrolysed or carbonized, without an uncontrolled ignition of the plastic residues being possible.
To further remove the residues which still remain afterwards, for example carbon residues, a second part of the cleaning cycle can be carried out, whereby air is admitted in the furnace room to further oxidize the organic residues.
The above-mentioned vacuum furnaces are disadvantageous, however, in that the total cleaning cycle necessarily takes long. The vacuum period must last long enough to make sure that the pyrolysis has come to an end before air is admitted in the furnace; otherwise, the sudden ignition of the plastics would overheat the object. Also, this first period lasts at least 60 minutes, and usually even longer. If one wishes to obtain an adequate cleaning result, a second period must be provided whereby air is admitted in the furnace. This period lasts at least 45 minutes and usually even longer.
A second disadvantage of such vacuum furnaces is that the melted-off plastics are collected separately. They usually cannot be re-used, and thus have to be removed as waste.